Spinoza’s account of reason in the Short Treatise has been largely neglected. That account, I argue, has at least four features which distinguish it from that of the Ethics: in the Short Treatise, (1) reason is more sharply distinguished from the faculty of intuitive knowledge, (2) reason deals with things as though they were ‘outside’ us, (3) reason lacks clarity and distinctness, and (4) reason has no power over many types of passions. I argue that these differences have a unified explanation, consisting of a principle that Spinoza accepts in both works and a central change in his views. The principle is that whatever we find in ourselves has more power over us than anything which comes from outside, and the change is in making the objects of reason common things or common notions. Understanding this, I claim, sheds important light on the psychological and epistemological motivations behind Spinoza's mature doctrines.